Southeast Minnesota is the sinkhole capital of the U.S.A. These strange holes in the ground are all over the place. Some have been here for as long as we can remember and some seem to pop up overnight. But we wanted to know, how do these sinkholes form?

What did we do?
We looked at three different sinkholes and measured their size with a tape measure. Then we stood in the center and stacked some tubes together to measure their height. The last thing we did was sketch what each sinkhole looked like. We also looked around the sinkholes for other features, like standing water and any special kinds of rocks.

What did we find out?
In the bottom of the first sinkhole, we found a little bit of limestone and a shell fossil. The hole wasn't very deep or wide, but we thought the limestone might be a clue to how the hole formed. The second hole was wider and deeper than the first and there was a lot of wood debris at the bottom. It reminded us of a kitchen sink drain, except this drain led underground. The last sinkhole was huge! It was so deep that we couldn't safely climb into it to measure the depth. We saw a lot of limestone shelves and another drain at the bottom. We explored some more and found that the drain led to an underground cave. From all these sinkholes we learned that limestone and water play a big part in making sinkholes. We figured that limestone layers must dissolve over time and then collapse to form a sinkhole.

What can you do?

You can get to the bottom of sinkholes too. Make your own model. You'll need a medium jar, sugar cubes, graham crackers, soil and water. From bottom to the top, stack layers of sugar cubes, graham crackers and a little soil. The sugar cubes represent the Earth's limestone. The graham crackers are soil, and the dirt is topsoil. Add drops of water to the topsoil, and then watch the limestone slowly crumble. The water will act like rain and will filter to the bottom. The sugar limestone layers will crumble and form a mini sinkhole.

Do you have sinkholes where you live? Why do some sinkholes have trees growing out of them and some don't? Have you passed sinkholes before and never paid attention to them? How far apart are they from each other?

In the old days, farmers filled sinkholes in their fields with garbage, old furniture and cars, but it began polluting their water supply. Sinkholes act like a drain. Rain flows through the Earth's layers, filtering pollution from the sinkhole down in to underground rivers that flow into our drinking water. How else is our water supply affected by pollution? How fast does the supply get contaminated? How would you test this?

Use this earth science investigation as a science fair project idea for your elementary or middle school science fair! Then tell us about it!